1. Field of the Invention
The present invention generally relates to a method of making tools and dies for producing a series of articles. More specifically, this invention relates to a method of making die shells from a model of predetermined dimensions, where the die shells are ultimately used for stamping, casting, molding, or forging a high volume of identical parts.
2. Description of the Prior Art
For many decades, forming dies have been used to produce various large high-volume parts for automobiles--including automobile fenders, hoods, doors, and interior panels for example. These parts often involve very tight tolerances and "Class-A" surfaces that demand use of precision dies. Class-A surfaces are those with which an automobile operator interacts, such as the outer surface of a hood or a door panel. Traditional precision dies are typically heavy, large metal bodies that are machined from blank stock, or cast to rough shape and then machine finished. Traditional dies demand a costly and time-consuming amount of precision machining of the entire die to achieve the tight final part tolerances.
Such dies are expensive for several reasons, but primarily because the dies are made of relatively expensive material and the traditional die making process requires use of a large volume of this material, much of which is wasted when machined away. In addition, the machining process requires use of large and expensive machining centers, expensive perishable tools, and substantial amounts of expensive time to finish the die. Some die designs also require use of equally costly and time-consuming hardening and finishing processes. Therefore, alternatives to the manufacture of traditional dies has been suggested over the years.
For example, in U.S. Pat. No. 3,463,035 to Bright there is disclosed a method of preparing die plates addressing the problem of cost prohibitive and time-consuming traditional dies. Bright teaches fabricating die plates by: providing a positive impression from a model; forming a coating of wax equal in thickness to the finished die plate over the positive impression; casting an undersized negative impression of hardenable material from the wax coated positive impression; explosion forming three sheets of die plate metal over the negative impression to conform therewith; and reinforcing the top and bottom die plates with an epoxy resin.
Unfortunately, this technology has many disadvantages. First, using a coating of wax to compensate for the finished part thickness is unreliable, since wax coating involves an intolerable amount of variation in thickness. Second, using a process that utilizes a hardenable material like epoxy resin, as Bright suggests, may be insufficient to withstand extremely high forming forces or processes that use high temperature forming techniques. Third, explosion forming can be dangerous and requires use of extremely robust and expensive equipment, and obviously a rather extraordinary degree of care. Fourth, the die plates must be removed from a negative impression before being reinforced by a substrate backing. This removal step will tend to distort the die plates thereby resulting in compromised die quality. Distortion may be minimized by sizing the die plates to withstand the forces incurred during the removal step. This, however, only increases the weight, expense, and time to make the dies.
A fifth disadvantage of the Bright reference is that the forming surfaces of each die plate are not produced directly off the corresponding forming surface of the mold, from which each die plate is made. Instead, the forming surfaces of each die plate are produced from opposite sides of a slave sheet of metal that is sandwiched between the die plates during the die forming process. This results in an inferior forming surface on the die plate since it is not a direct descendant of the "master" surface of the model. The forming surface of the mold is also not produced directly off the master surface of the preceding pattern or model. Instead, the forming surface of the mold is produced by an outer surface of a wax layer that is intermediate the mold and a preceding pattern or negative impression. Therefore, the forming surfaces of each die of the Bright reference cannot be traced directly back to the model master surface. It can only be said they trace indirectly back through the medium of a wax layer and a slave sheet, to the model master surface. Furthermore, the forming surface of each die plate is at least four steps removed from the model master surface.
In another example, U.S. Pat. No. 4,088,046 to Severinsson, there is disclosed a method of producing forming tools addressing the problems of die deformation from prior art die preparation. Severinsson teaches fabrication of a mould shell forming tool by: providing a master pattern; thermal spraying a soft first layer of metal on either side of the master pattern; thermal spraying a harder second layer of metal over the first layer; and bonding a support material to the back sides of the mould shells. Likewise, U.S. Pat. No. 5,337,631 to Singer et al., teaches a similar method of producing tools and dies. Singer et al. teach fabrication of a die by providing a pattern, electroforming the die by electroplating the surfaces of the pattern to form a shell, then filling the shell by thermal spraying or peening. Thermal spraying can be relatively inaccurate due to variations in thickness stemming from operator inattention. Therefore, the Singer et al. process requires extraordinarily sophisticated metal deposition technology that tends to be relatively expensive and requires specialized expertise.
From the above, one can see several attempts that were made over the years to replace the inefficient, expensive, and time-consuming traditional methods of machining or casting dies. What is needed, therefore, is a novel method for producing accurate dies that is solid-state, less expensive, able to withstand severe press forces, and automatically compensates for actual part thickness.